1. Field of the Invention
The present invention relates to an apparatus and method for reducing the nitrogen oxide emissions from ga turbines. More specifically, it relates to the introduction of gaseous fuel to the intake air utilized in a gas turbine.
2. Description of the Prior Art
In the combustion of fuels in gas turbines, oxygen from the air may combine with nitrogen from the air to form nitrogen oxides. This reaction occurs to a greater extent at higher operating temperatures. Additionally, some oils that are fired in gas turbines have fixed nitrogen, a part of which generally reacts with oxygen from the air to form nitrogen oxide. This reaction occurs even at relatively low combustion temperatures.
Production of this nitrogen oxide is generally regarded as undesirable. There are numerous government regulations which limit the amount of nitrogen oxide which may be emitted into the atmosphere from a gas turbine. Furthermore, the rapid cooling of the combustion products from a gas turbine causes much of the nitric oxide (NO) to be oxidized to nitrogen dioxide (NO.sub.2). Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions from gas turbines.
A gas turbine is designed to burn oil, natural gas or other fuels. The fuel is mixed with air in a combustor can. As the air and fuel enter the combustion area, combustion occurs in a flame zone where the temperature is normally well above 3000.degree. F. The combustion products are cooled to around 2000.degree. F. by mixing with excess air which enters through a register. The combustion products, diluted with excess air, exit the can and go through duct work to an expansion turbine where they drive turbine blades. These blades turn a shaft which in turn drives a compressor and a generator. After the power turbine extracts as much energy as possible from the combustion products, they are exhausted to the atmosphere through a stack.
During the combustion of the fuel and air at very high temperatures, some of the oxygen and nitrogen from the air combine to form nitrogen oxide. In addition, a large fraction of fixed nitrogen, which is present in petroleum, coal, and some other fuels, combines with oxygen from the air to form nitrogen oxide.
Numerous attempts have been made to develop devices and processes which reduce the nitrogen oxide emissions from these gas turbines. One such approach is the injection of water into the flame zone in order to reduce the peak flame temperatures and thus reduce the formation of nitrogen oxide. This process has had some technical and commercial success, but it suffers from at least three disadvantages. First, the water injection reduces the efficiency of the process, making it necessary to burn more fuel to obtain the same amount of electrical power. Second, the water contains suspended and dissolved solids, such that even with relatively clean water, particulate emissions are increased. These particulate emissions may also be considered undesirable. Third, the water injection may increase the carbon monoxide emissions.
Other proposed modifications include changes to the combustor can in order to reduce NO.sub.x emissions. These changes are intended to add air to the combustor can in greater than stoichiometric quantities, so that combustion occurs at a lower temperature. In diffusion flames, combustion occurs at stoichiometric mixtures. The flame temperature is therefore maximized even though the quantity of air surrounding the combustion is large enough to dilute the flame, causing it to burn at a lower temperature. Excess air mixes with the combustion products at a very high temperature and a portion of the oxygen contained therein reacts to form NO.
In gas turbines, the combustion air or working fluid is compressed to very high pressures which causes the temperature to become very high. Fuel is introduced simultaneously with the hot, compressed air into the combustor can. The fuel burns in the air, raising the temperature and volume further. The combustion products are then introduced into the power turbine where they can expand. Generally, a great excess of air flowing around the combustor can is used to limit the peak temperature. Holes in the can allow some air to enter around the periphery of the can to provide further cooling. This air is sometimes induced to flow as a film on the inside perimeter of the can. Other air flows along the outside of the can. Both air flows cool the can and in turn become heated. This air then enters the burner as very high temperature combustion air. Various improvements with alternative holes cut in the can have been tried, but have met with only limited success.
Burner modifications have also met with limited success. Since each can has only one burner, it is not possible to run the burners in a non-stoichiometric mode. Although each gas turbine may have several combustor cans, each can must have an excess of air to cool it properly. It is thus not possible to operate some cans in a fuel rich mode and others in an air rich mode, and then mix the products to complete the combustion and produce the desired temperature for the turbine.
Some of these approaches have been successfully utilized on boilers, but are not practical in gas turbines. Other techniques for reducing NO.sub.x emissions, such as reduced air preheat, increased primary furnace size, flue gas recirculation, improved heat transfer, in-furnace NO.sub.x reduction, and thermal DeNO.sub.x processes, which have been found to be effective in furnaces are also not practical in gas turbines.
There is, therefore, a need for a method and apparatus to reduce NO.sub.x emissions from gas turbines.